Why does gas light when it is passed through a flame?
How is gas flammable?

Allan Blackman, a chemist at the University of Otago, responded.

You will have seen that the gas used in such common appliances as gas
heaters, gas ovens or gas camping cookers catches fire if you apply a spark
or a flame. This is in fact a chemical reaction called oxidation, in which
the gas reacts with oxygen from the air.

A gas heater uses "natural" gas, which is a mixture of chemical compounds
called hydrocarbons. When these compounds undergo oxidation they are converted
to water and carbon dioxide, and the reaction liberates large amounts of heat.
However, these reactions usually require an initial input of energy (in the
form of a spark or flame) in order for them to proceed. This is why
(thankfully!) natural gas does not spontaneously ignite in air. Hydrogen gas
will oxidise explosively in air when it is ignited (ask your science teacher
for a demonstration of this - it's impressive!), and this was the cause of
the a disaster in 1937, when the hydrogen-filled German airship "Hindenburg"
caught fire during landing. Since then, airships have been filled with helium,
an inert gas that does not undergo oxidation.